1. Field of the Invention
The present invention relates to a secondary battery. More particularly, the present invention is concerned with a secondary battery comprising an organic electrolytic solution having disposed therein a positive electrode comprised of a lithium-containing composite metal oxide as a cathode active material and a negative electrode comprised of a carbonaceous material as an anode active material, wherein the organic electrolytic solution has a water content of from 5 ppm to 450 ppm, so that the secondary battery has excellent current efficiency, cycle characteristics, storage characteristics and safety.
2. Discussion of Related Art
In recent years, various non-aqueous type secondary batteries have been proposed as compact, light weight batteries to be advantageously substituted for conventional acid-lead batteries and nickel-cadmium batteries. Among these proposed secondary batteries, new type secondary batteries using a composite metal oxide comprised mainly of Li and Co as a cathode active material and using a carbonaceous material as an anode active material, have been attracting attention in the art. Such new type secondary batteries are disclosed in, for example, Japanese Patent Application Laid-Open Specification Nos. 62-90,863 (corresponding to U.S. Pat. No. 4,668,595), 63-121,260, and 3-49,155 (corresponding to U.S. Pat. No. 4,943,497).
In conventional non-aqueous type secondary batteries, metallic lithium or a lithium alloy has been used as an anode active material. Such conventional secondary batteries using metallic lithium or the like as an anode active material, are satisfactory with respect to compactness in size and lightness in weight, but have various problems in practical use thereof, such as a lowering of cycle characteristics and a lowering of storage characteristics due to the deposition of dendrites, an occurrence of internal short-circuiting due to the breakage of a separator by deposited dendrites, and a safety problem ascribed to the high reactivity of metallic lithium.
In contrast, with respect to the above-mentioned new type secondary batteries using a carbonaceous material as an anode active material, no deposition of dendrites occurs, so that excellent cycle characteristics and storage characteristics can be enjoyed, and, in addition, the carbonaceous material does not have a high reactivity, unlike metallic lithium, so that extremely high safety can be achieved. Especially, it has been expected that the combined use of a carbonaceous material anode and a lithium-containing composite metal oxide cathode would provide a secondary battery exhibiting high voltage and high capacity.
However, actually, such non-aqueous type secondary batteries using a lithium-containing composite metal oxide as a cathode active material in combination with a carbonaceous material as an anode active material, frequently suffer not only from various problems in performance, such as a lowering of current efficiency and a lowering of cycle characteristics, but safety problems also develop due to the occurrence of a rise in internal pressure.